This is a major release with several key additions, most notably being
the support for custom, mpv-style "user shaders" (.hook), giving us
access to a large variety of pre-existing user shaders such as RAVU,
FSRCNNX, Anime4K, SSimSuperRes, KrigBilateral, NNEDI3, and more.

In addition to this, major additions include a completely refactored and
fixed AV1 grain generation shader, support for Vulkan versions higher
than 1.0, support for GPU-based timers, and improved interop APIs for
both Vulkan and OpenGL, and new and improved aspect ratio handling.

Finally, this release also brings with it a major change to the way HDR
and SDR content are mapped between each other, including a new
tone-mapping function based on the industry-standard ITU-R BT.2390 EETF.

- add `pl_swapchain_hdr_metadata`, to set HDR metadata on supported
  swapchains (currently only vulkan with `VK_EXT_hdr_metadata`)
- add support for vulkan versions higher than 1.0, communicated via the
  new fields `api_version` and `max_api_version`
- add support for GPU-assisted validation and best practices layers, via
  the new field `pl_vk_inst_params.debug_extra`
- add helper functions for working with `pl_rect`s, including new aspect
  ratio handling helpers (`pl_rect2df_aspect_*`)
- add field `pl_vulkan_params.device_uuid` to allow choosing the vulkan
  device by its UUID
- add function `pl_vulkan_hold_raw`, to hold images without actually
  transitioning its layout and access mode
- add function `pl_vulkan_import`, to allow directly re-using an
  existing VkDevice rather than creating a new one; this requires
  communicating metadata about how the device was created
- add field `pl_vulkan_params.features` to allow loading optional extra
  device features at device creation time
- add support for mpv-style custom user shaders (.hook), using the set
  of functions in `<libplacebo/shaders/custom.h>`
- add `pl_render_high_quality_params`, enabling debanding and EWA
- add `pl_timer` GPU resource type and associated API functions,
  allowing the GPU execution time of shaders and texture transfer
  operations to be measured directly
- add `PL_SHADER_SIG_SAMPLER`, allowing generated sampling shaders to
  directly accept the sampler to use as function parameters
- add `pl_image_set_chroma_location` to automatically apply the correct
  chroma location to any subsampled planes
- add `PL_TONE_MAPPING_BT_2390`, a tone mapping function based on the
  EETF from ITU-R Report BT.2390 (and make it the default)
- add `pl_peak_detect_params.overshoot_margin` to help combat clipping
  on certain types of rapid scene fade-ins
- add `pl_sampler_type` to allow encoding non-standard sampler types
  such as sampler2DRect, and also generalize samplers to allow e.g.
  usampler2D or isampler3D
- add `pl_opengl_wrap` and `pl_opengl_unwrap`, to allow directly mapping
  between OpenGL textures and the `pl_tex` abstraction

- deprecate `pl_image.width/height`, which are now inferred
  automatically from the actual planes
- `pl_vulkan_wrap` now takes a `pl_vulkan_wrap_params` struct instead of
  directly accepting its parameters, including new fields `sample_mode`
  and `address_mode` to configure the created sampler
- change `pl_dispatch_compute` to allow optionally passing in a
  simulated framebuffer width/height, which will be used to translate
  vertex attributes (if any)
- undefine disabled `config.h` features, instead of defining them as 0
- remove debanding from `pl_render_default_params`
- refactor HDR<->SDR mapping; PL_COLOR_REF_WHITE has been removed and
  replaced by PL_COLOR_SDR_WHITE (203 cd/m^2) and PL_COLOR_SDR_WHITE_HLG
  (75% HLG), respectively
- completely refactor pl_shader_av1_grain`, which now samples directly
  from the passed texture rather than requiring the color be pre-sampled
- `pl_render_image` now infers the image primaries based on resolution,
  rather than always hard-coding `PL_COLOR_SPACE_UNKNOWN` as BT.709
- change `pl_render_target.dst_rect` from `pl_rect2d` to `pl_rect2df`,
  allowing more accurate aspect ratio handling, and correctly compensate
  for subpixel scaling ratios
- require `python3-mako` as a dependency of the `vulkan` feature
- `pl_chroma_location_offset` now treats `PL_CHROMA_UNKNOWN` as
  `PL_CHROMA_LEFT`, the de-facto standard chroma location
- the default value of `pl_color_map_params.tone_mapping_algo` is now

Fixes and performance improvements:
- fix shader generation when the GLSL version is explicitly overridden
- properly mark some shader failures (`pl_shader_is_failed`)
- fix texture invalidation on OpenGL
- correctly respect `pl_swapchain_frame.flipped` in
- correctly validate descriptor uniqueness in `pl_pass_create`
- skip redundant matrix multiplication in `pl_shader_encode_color`
  wherever possible
- work around driver bugs w.r.t out-of-order buffer offsets by sorting
  all buffer variables by offset
- fix edge cases in vulkan swapchain usage flag checks
- fix excessive CPU usage in `pl_tex_download`
- reduce the number of unnecessary GPU flushes caused by `pl_buf_poll`
- fix issue where blending did not work on some drivers (e.g. nvidia)
- make the framebuffer discard check more aggressive
- fix computation of anti-aliased resizable orthogonal filters, e.g.
  when downscaling using `pl_filter_lanczos`
- fix external image memory barriers for exclusive mode images
- fix failure path of `pl_swapchain_submit_frame`
- fix various GLSL compatibility issues with av1 grain generation
- reduce maximum vulkan memory allocation slab size to conform to AMD
- fix build error when lcms is not available
- fix double-application of texture scale for e.g. 10-bit content when
  using separable scalers
- fix a multitude of bugs affecting av1 grain generation, especially for
  chroma planes
- fix segfault on vulkan device oom
- fix invalid shader generation on some platforms
- fix a multitude of bugs, edge cases and subtle off-by-ones related to
  chroma scaling and plane alignment
- add fallback code for edge case w.r.t chroma scaling and gpu resource
- correctly load VK_KHR_swapchain in all circumstances that require
  accessing its functions
- minimize fbo usage inside `pl_renderer`, by re-using unused fbos
- tweak the work group size for polar scaling to perform better on
  modern GPUs (tested on RDNA)
- transparently upgrade fragment shaders to compute shaders on
  environments with async compute
- pick a more reasonable size for the dummy gpu's `max_group_threads`
- forbid 10-bit linear transfer functions from vulkan swapchains
- fix segfault when re-executing previously failed shaders
- fix swapchain creation errors on GLES 2
- explicitly mark all shader resources as non-aliased
- correctly specify shader storage buffers as coherent for shaders that
  require them
- fix various memory barrier synchronization issues for opengl
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